Chemical vapor deposition (CVD) is the method that has been used to deposit diamond thin films. This process involves chemical reaction between two gases or between a volatile compound and gases to produce solid material that deposits atomistically onto a substrate. The various techniques that have been developed to accomplish chemical vapor deposition fall generally into three categories: thermally assisted CVD, plasma assisted CVD and combustion flame diamond deposition. Variations and combinations of these techniques have also been used. The various CVD processes are carried out using a carbon-containing gas such as CH.sub.4, C.sub.2 H.sub.2, acetone, CO and CO.sub.2. Typically the carbon-containing gas has been used in a hydrogen mixture with the percentage of hydrogen being 95% or higher. Less-dilute mixtures with hydrogen, mixtures with other gases and pure carbon-containing gas have also been used, see e.g., P. K. Bachman et al., Diamond and Related Materials 1, 1 (1991) for a review of CVD.
Microwave plasma enhanced CVD has proven particularly useful for achieving diamond deposition at relatively low temperatures. In microwave plasma enhanced chemical vapor deposition, glow discharge plasmas are sustained within the chamber where the chemical vapor deposition reactions occur. The presence of the plasma promotes the decomposition of the gas molecules into ions, electrons, atoms and molecules so that reactions occur at lower temperatures when plasma enhanced chemical vapor deposition is used in contrast to the often prohibitively high temperatures required when no plasma is used.
Microwave plasma enhanced CVD of diamond is generally carried out with a mixture of a small amount of a carbon-containing gas such as CH.sub.4, C.sub.2 H.sub.2, CO and CO.sub.2 in hydrogen. During the process the carbon-containing gas is decomposed at substrate temperatures of about 700.degree. C. to about 900.degree. C. in the presence of hydrogen to form diamond. Both diamond and graphite phases may deposit simultaneously. It is important to either prevent the graphite from forming or to selectively remove the graphite leaving behind the diamond. This can be accomplished by supersaturating the plasma with atomic hydrogen. Typically, a diluted mixture of about 0.5 to about 2% of CH.sub.4 in hydrogen is used.
Diamond nucleation rate is a concern in CVD techniques and various attempts have been made to enhance the nucleation rate. It is well-known that no matter what material is used as the substrate, scratching the surface of the substrate with an abrasive material, particularly diamond powder, greatly enhances the diamond nucleation rate. See, e.g., J. C. Angus et al., Proc. Second Int'l. Symp. on Diamond, The Electrochemical Society, Inc., Pennington, N.J., 91-8, 125 (1991). Typically, this scratching is accomplished by rubbing the surface to be coated with diamond powder or grit. U.S. Pat. No. 5,075,094 illustrates this (and also notes that diamond seems to nucleate strongly on areas of substrates with hydrocarbon contamination). G. H. Johnson et al., Diamond and Diamond-Like Materials Synthesis, MRS Symposia Proc., EA-15, 99 (1988), also disclose that scratching substrates of silicon with various abrasives (particularly diamond powder) greatly enhances the diamond nucleation rate and growth. This publication also discloses that seeding silicon substrates with various hard materials (particularly diamond powder) greatly enhances the diamond nucleation rate and growth. However, the small surface dimensions of articles such as fibers preclude the effective use of conventional rubbing methods. U.S. Pat. No. 5,374,414 to Moorish et al. discloses coating filamentous substrates with diamond by CVD to make diamond or diamond-coated filaments (e.g., diamond tubes or diamond fibers). For high nucleation, substrate filaments are scratched by placing them in an ultrasonic cleaner containing diamond grit.
Another problem which can occur during CVD deposition of diamond onto a shaped article, particularly microwave enhanced CVD, involves substrate deterioration. Atomic hydrogen generated in the microwave plasma is believed to aid diamond growth and selectively removes any graphite that forms. M. Frenklach, Proc. of 2d Int. Symp. on Diamond Materials, Electrochemical Society, Proc. Vol. 91-8, 142 (1991), discusses the preferential etching of graphite over diamond by hydrogen atoms during diamond deposition. Both diamond and graphite are formed simultaneously but graphite is destroyed by reactions with H atoms while diamond is not. The atomic hydrogen also tends to etch the substrate (see, e.g., G. H. Johnson et al., Diamond and Diamond-like Materials Synthesis, MRS Symposia Proc., EA-15, 99 (1988)) and whether this etching becomes a problem depends upon the morphology and material of the substrate as well as the diamond nucleation rate. This etching by hydrogen can be especially troublesome when trying to deposit a diamond coating on a shaped article having small dimensions. Examples of such shaped articles are fibers, cylindrical or spherical particles and powder particles. A graphite substrate is among the most difficult since the atomic hydrogen readily interacts with the graphite to form volatile hydrocarbons which are removed from the surface. As a result a graphite shaped article, for example, a graphite fiber, can be etched away before the diamond coating forms on the surface. In one embodiment, A. A. Moorish et al., U.S. Pat. No. 5,374,414 disclose growing diamond tubes or solid diamond fibers on graphite fibers. Their graphite fibers which normally would be immediately etched away by the atomic hydrogen, are passivated by coating the graphite with a passivating material such as copper, and diamond then is deposited on the copper-coated graphite fibers. As the diamond is deposited on the surface of the copper-coated graphite fibers, the hydrogen simultaneously etches away the graphite, leaving a tubule of diamond with an inner copper coating. J. H. Lemelson, U.S. Pat. No. 4,859,493, discloses a method for forming diamond coatings on fibers, spheroids, etc. in which diamond is deposited on objects which are in free fall, fluidized or tumbling in a vapor or gas of carbon-containing molecules and hydrogen. The use of extra hydrogen to minimize graphite formation is discussed. The problem of graphite etching by hydrogen when the object to be coated is graphite is not addressed. Accordingly, there remains a need for effective methods of CVD deposition of diamond onto articles made of graphite.
SUMMARY OF THE INVENTION
This invention provides a process for the chemical vapor deposition of diamond onto a shaped graphite article, and diamond-coated shaped graphite articles which may be produced by that process. The process comprises the steps of placing the shaped graphite article in a suspension of diamond powder in a liquid (e.g., methanol); agitating this suspension containing the shaped graphite article; and removing the shaped graphite article from the suspension and drying it prior to depositing a diamond coating on the shaped graphite article by chemical vapor deposition. This process provides enhanced diamond nucleation on the article surface during CVD, and is especially useful for the chemical vapor deposition of diamond onto shaped graphite articles having at least two dimensions which are small and/or shaped articles made of materials which are susceptible to etching during CVD (e.g., graphite fibers). This invention provides diamond-coated graphite fibers which have diameters less than about 100 microns.